Multijunction solar cell employing extended heterojunction and step graded antireflection structures and methods for constructing the same

ABSTRACT

Material and antireflection structure designs and methods of manufacturing are provided that produce efficient photovoltaic power conversion from single- and multi-junction devices. Materials of different energy gap are combined in the depletion region of at least one of the semiconductor junctions. Higher energy gap layers are positioned to reduce the diode dark current and enhance the operating voltage by suppressing both carrier injections across the junction and recombination rates within the junction. Step-graded antireflection structures are placed above the active region of the device in order to increase the photocurrent.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/719,811, filed Mar. 8, 2010, entitled MULTIJUNCTION SOLAR CELLEMPLOYING EXTENDED HETEROJUNCTION AND STEP GRADED ANTIREFLECTIONSTRUCTURES AND METHODS FOR CONSTRUCTING THE SAME, now U.S. Pat. No.8,895,838, which claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/293,469, filed on Jan. 8, 2010, entitledEFFICIENT SOLAR CELL EMPLOYING MULTIPLE ENERGY-GAP LAYERS ANDLIGHT-SCATTERING STRUCTURES AND METHODS FOR CONSTRUCTING THE SAME, theentire disclosure of each of which applications is expresslyincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to semiconductor-based photovoltaic energyconverters, also known as “solar cells,” and to the design andfabrication of the same.

BACKGROUND OF THE INVENTION

With appropriate electrical loading, photovoltaic solid statesemiconductor devices, commonly known as solar cells, covert sunlightinto electrical power by generating both a current and a voltage uponillumination. The current source in a solar cell is the charge carriersthat are created by the absorption of photons. These photogeneratedcarriers are typically separated and collected by the use of PN or PINjunctions in semiconductor materials. The operational voltage ofphotovoltaic devices is limited by the dark current characteristics ofthe underlying PN or PIN junction(s). Thus improving the power outputperformance of any solid state solar cell generally entailssimultaneously maximizing absorption and carrier collection whileminimizing dark diode current.

Detailed balance calculations are typically used to compute the ideal,limiting performance of semiconductor solar cell devices (see forexample, C. H. Henry, Limiting Efficiencies of Ideal Single and MultipleEnergy-gap Terrestrial Solar Cells, J. Appl. Phys., vol. 51, pp.4494-4500, August 1980). Two fundamental assumptions are traditionallymade in these theoretical calculations. First, it is assumed that thediode dark current is limited by radiative recombination, and that theradiative recombination rate is set by the energy gap of thesemiconductor material used to fabricate the device. Second, all of thephotons in the incident spectrum with energy above the energy gap of thedevice material are assumed to create a charge carrier pair that issuccessfully separated and collected. In practice, neither of theseassumptions is achieved. The dark current, and thus the operatingvoltage, of single junction homojunction solar cells and subcells aretypically limited by non-radiative recombination mechanisms such asspace charge recombination and majority carrier injection. Non-radiativerecombination processes along with reflection losses also limit thecurrent generating capability of single junction devices. Thus practicalsingle junction solar cells have yet to reach the performance levelspredicted by detailed balance calculations.

In recent years, multijunction solar cell structures have broken theShockley-Queisser limit on solar cell performance derived from detailedbalance calculations. Multijunction structures employ several differentenergy-gap materials, typically in separate PN junctions combined withina monolithic III-V material structure. Compared to state-of-the-artsingle junction GaAs solar cells, two- and three-junction III-V solarcells have roughly one half the current output, but benefit from agreatly increased voltage, which can be a factor of 2.5 to 3× higher,depending on the number junctions used and the individual properties ofeach junction subcell.

Even with the record breaking efficiency achieved with III-Vmultijunction solar cells, there remains keen interest in furtherimproving the power output of these devices for both space andterrestrial applications. Therefore, it is desirable to provide fordesigns that can effectively suppress dark currents in each of theindividual junction subcells employed in multijunction devices.Moreover, it is also desirable to provide design strategies andprocesses that can maximize the photocurrent generating capability ofthe limiting subcell within each multijunction structure.

SUMMARY OF THE INVENTION

This invention overcomes the disadvantages of the prior art by providinga multijunction solar cell structure and method of manufacturing thatincludes two design elements that separately, or in combination, canincrease the power output of semiconductor solar cells. When fullyfunctionalized, an illustrative embodiment combines the two sets ofdesign elements together to increase both the voltage and current outputof multijunction solar cells. Moreover, this invention providesenhancements to both the voltage and current generating capability ofthe III-V multijunction photovoltaic devices.

The first design element relates to the material structure of the activeregion of one or more of the subcells within a multijunction devicewhere photo-generated carriers are created and separated. Notably, thebasic active region structure of at least one of the subcells consistsof a PN or PIN junction which contains materials of different energy gapwithin the junction depletion region. Moreover, a novel feature of thedesign is the positioning of the different energy gap material withinthe active region. In an illustrative embodiment, a wider energy gapbarrier layer is positioned within the depletion region adjoining theemitter layer in order to suppress carrier injection across thejunction. In addition, wider energy gap material can be located withinthe depletion region in the zone of enhanced space charge recombination,where the injected electron and hole concentrations are comparable. Inan illustrative embodiment, the subcell structure positions at least oneof the transitions from the narrowest energy gap material in activeregion to a wider energy gap material so that that it remains within thedepletion region over a wide range of bias levels, even at forward biaslevels appropriate for photovoltaic power generation.

The second design element relates to the application of antireflectionlayers above the active region of the device. The purpose of theseantireflection structures is to maximize the number of incident photonsthat are directed into the device active region. In particular, multiplelayers of material with index of refraction intermediate between that ofthe top subcell material and air can be formed on the top most devicesurface facing the sun. These lower index of refraction layers canconsist of both epitaxial semiconductor material and optical thin filmmaterial. The exact thickness and index of refraction of the layer inthe antireflection structure can be adjusted to minimize reflectionlosses over a broad spectrum of photon wavelengths and angles ofincidence.

Illustratively, efficient photovoltaic devices both maximize thecreation and collection of photo-generated carriers and enhance thevoltage at which photo-generated carriers are extracted.

In an illustrative embodiment, a junction solar cell device defining asubcell-based construction with one or more subcells comprises at leastone subcell containing an extended heterojunction. The heterojunctionstructures defines a semiconductor PN or PIN junction having an emitterand a base, in which the emitter consists of a material with a higherenergy gap than an energy gap of a material that defines the base. Anextended heterojunction structure also comprises at least one layerlocated in a depletion region adjacent to the emitter. Such layerincludes a material with a higher energy gap than the energy gap of thematerial that defines the base.

In another illustrative embodiment, a junction solar cell devicedefining a subcell-based construction with one or more subcellscomprises a broadband, step-graded antireflection structure locatedabove the topmost subcell. The construction can further includeepitaxial semiconductor layers constructed and arranged to reduce theindex of refraction at a top surface of the solar cell device.

In another illustrative embodiment, a method of manufacturing amultijunction solar cell provides a subcell-based structure having atopmost subcell. A step-graded antireflection structure is deposited ona top of the topmost subcell. The subcell can include an extendedheterojunction, defined as a semiconductor PN or PIN junction having anemitter and a base, in which the emitter and a layer located in adepletion region adjacent to the emitter consists of a material with ahigher energy gap than an energy gap of a material that defines thebase. The method can also further comprise varying an index ofrefraction of the anti-reflection structure by controlling a thicknessand a composition of top epitaxial semiconductor layers and bydepositing multiple layers of TiO₂- and SiO₂-based optical coatings.Additionally, the method can include forming a top optical coatingstructure by co-sputtering and oblique angle deposition adjacent to thetop of the topmost subcell. The method can also include, firstdepositing buffer layers on a Ge substrate and next epitaxially growingin order (a) a GaAs-based lower subcell with an extended heterojunction;(b) a tunnel junction structure; (c) a top InGaP-based subcell with anextended heterojunction; and (d) a top GaAs contact layer; gradedrefractive index layer, after selective removal of the GaAs contactlayer and the application of metallic contacts and gridlines. Likewise,the method can comprise (a) growing an epitaxial structure inverted on aGaAs substrate; then (b) removing epitaxial layers; (c) forming metalliccontacts and gridlines; and (d) depositing a graded refractive indexantireflection coating thereover.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be readily understood by the following detaileddescription in conjunction with the accompanying drawings, wherein likereference numerals designate like structural elements, and in which:

FIG. 1 is a diagram of the semiconductor energy band versus position forthe material structure of a conventional homojunction solar cellaccording to a prior art arrangement;

FIG. 2 is a diagram of the semiconductor energy band versus position forthe material structure of a heterojunction solar cell according to aprior art arrangement;

FIG. 3 is a diagram of the semiconductor energy band versus position fora material structure illustrating an extended heterojunction solar cellwhich can minimize diode dark current;

FIG. 4 is a diagram of the semiconductor energy band versus position fora material structure illustrating an extended heterojunction solar cellwith a barrier layer which can further minimize diode dark current;

FIG. 5 is a diagram of the energy band versus position for a materialstructure of a semiconductor solar cell illustrating an extendedheterojunction solar cell with a barrier layer using InGaP-basedmaterials typically employed in the top subcell of III-V multijunctionsolar cells according to various embodiments;

FIG. 6 is a diagram of the energy band versus position for a materialstructure of a semiconductor solar cell illustrating an extendedheterojunction solar cell with a barrier layer using GaAs-basedmaterials typically employed in the middle subcell of III-Vmultijunction solar cells according to various embodiments;

FIG. 7 is a diagram of the energy band versus position for a materialstructure of a semiconductor solar cell illustrating an extendedheterojunction solar cell with a barrier layer using InGaAs-basedmaterials typically employed in the lower subcell of III-V multijunctionsolar cells;

FIG. 8 is a graph showing index of refraction versus position for astep-graded antireflection coating on a top InGaP subcell, in accordancewith illustrative embodiments, to enhance optical transmission into theactive regions of the underlying III-V multijunction solar cell;

FIG. 9 is a graph showing index of refraction versus position for astep-graded antireflection coating on a top InGaP subcell, in accordancewith another illustrative embodiment in which epitaxial material is alsoemployed in the step-graded antireflection structure;

FIG. 10 is a schematic side cross section of a multijunction solar celldevice according to an illustrative embodiment incorporating extendedheterojunctions in both the top and lower epitaxial subcells as well asa step-graded antireflection coating; and

FIG. 11 is a schematic side cross section of an inverted multijunctionsolar cell device according to an illustrative embodiment with a top,middle, and bottom subcell incorporating extended heterojunctions and astep-graded antireflection coating.

The drawings are not necessarily to scale, emphasis instead being placedupon illustrating embodiments of the present invention.

DETAILED DESCRIPTION

FIG. 1 details a conventional arrangement for an epitaxial subcell in atypical III-V multijunction solar cell consisting of a top window 100,an emitter 120, base 140, and back surface field layer 160. As in any PNor PIN semiconductor junction, a depletion region 180 of high electricfield forms between the emitter and base layers. The window 100 and backsurface field 160 layers are designed to both confine photogeneratedcarriers and maximize optical transmission into underlying layers. In aconventional homojunction subcell, the energy gap of emitter 120 andbase 140 materials is identical, and thus the depletion region consistsof a single type of semiconductor material. The depicted layers in thisstructure, and other layered structures depicted herein, are alignedwith associated graphs of energy versus position (depth) so as toprovide further understanding of the performance of the structure.

Replacing the emitter layer with a higher energy gap material and thusforming a heterojunction subcell, as depicted in FIG. 2, can providevarious benefits for multijunction photovoltaic devices, as has beentaught by, for example, Fatemi et al., U.S. Pat. No. 7,553,691. Theoriesmay vary with respect to the physical principals that govern, but ingeneral, the larger emitter 220 energy gap can reduce thediffusion-driven injection of majority carriers from the base 240 intothe emitter 220. However, other significant diode dark currentcomponents, such as space charge recombination within the depletionregion and carrier injection from the emitter 220 into the base 240, arenot significantly improved with a standard heterojunction design. Thephysical boundary between higher energy gap and lower energy gapmaterial in prior art heterojunction subcells, as depicted in FIG. 2,also corresponds to the physical boundary between n-doped to p-dopedsemiconductor materials.

The layered structures and associated energy band graphs depicted inFIGS. 3 and 4, which are simplified for ease of understanding,illustrate the essential elements of a material structure that cansuppress the dark current of a photovoltaic device. The basic materialstructure consists of a PN junction (depicted in alignment to the sideof the graph, and representing the physical layered material), with anemitter 320/420, a base 340/440, and a depletion region 380/480. Thedepletion region 380/480 is distinguished by the presence of a built-inelectric field, illustrated in FIGS. 3 and 4 by a non-zero slope in theconduction (E_(c)) and valence bands (E_(v)), induced by thejuxtaposition of p-type and n-type semiconductor material. The thicknessof the depletion region can be increased by heavily doping one side ofthe junction (the emitter in FIGS. 3 and 4), and by addingunintentionally doped material between the p-type and n-type material,forming what is commonly referred to as a PIN junction. As with manytypical solar cell designs, window 300/400 and back surface field360/460 layers can be included above and below the PN junction.Illustratively, photo-generated minority carrier holes in the emitter320/420 and base 340/440 adjacent to the depletion region 380/480diffuse to the depletion region in the structure depicted in FIGS. 3 and4. However, compositionally graded emitter 320/420 and base 340/440material can be employed as well in an alternate embodiment (not shown),the implementation of which should be clear to those of skill in theart.

The arrangement of the depletion region 380/480 incorporates severalnovel features in accordance with illustrative embodiments contemplatedherein. First, higher energy gap material is inserted into the depletionregion 380/480 adjacent to the emitter 320/420 to form a higher energygap (E_(g)) depletion region layer 330/430. In contrast to prior art,the physical boundary between the different energy gap materials isoffset from the physical boundary between the n- and p-type materials.In one embodiment, depicted in FIG. 3, the higher energy gap depletionregion layer 330 is a single material composition matching that of theemitter 320, but is either undoped or with a dopant type matching thebase to ensure that this higher energy gap layer lies entirely withinthe depletion region. In another embodiment, depicted in FIG. 4, thehigher energy gap depletion region layer 430 consists of severaldifferent material compositions. In particular, a barrier layer 425 isplaced within the high energy gap depletion region layer 430 close tothe emitter 420. The purpose of the barrier layer 425 is to provide abarrier to the diffusion of majority carriers out of the emitter 420.

By way of further background, for photovoltaic applications, energy-gapdifferences at heterointerfaces within the device structure can act asunwanted barriers to the extraction of photo-generated carriers.However, field-assisted thermionic emission and tunneling arewell-established mechanisms by which carriers can escape from apotential well (see for example, by way of useful backgroundinformation, A. Alemu, J. A. H. Coaquira, and A. Freundlich, Dependenceof Device Performance on Carrier Escape Sequence in Multi-Quantum-Wellp-i-n Solar Cells, J. Appl. Phys., vol. 99, no. 084506, May 2006). Whilevarious theories of operation may be applicable, in general the basematerial 340/440 employs narrower band gap material, and the interfacewith larger energy gap material depicted in FIGS. 3 and 4 occurs in aregion containing a non-zero built-in electric field.

In general, the dimension of the wide band gap material 330/430 andbarrier layer 425 should be as small as possible, while still providinga significant barrier to majority carrier injection into the base, andwide enough to encompass the region of enhanced space chargerecombination. The total thickness of the narrower band gap basematerial 340/440 should be as large as possible to allow adequate photonabsorption, while the position of the heterojunction between narrow andwide energy gap material is constrained by the need to avoid the regionof enhanced space charge recombination and the need to position anyabrupt heterointerface in a region of high built-in electric field.Given these constraints, the exact dimensions can be optimized for anygiven material system with basic experimentation clear to one of skill,including varying the thickness of the barrier layer 425 and the higherenergy gap depletion region layer 330/430. An exemplary set of valuesfor initiating such experimentation is 75 nm for the total high energygap depletion region layer 330/430 thickness, and a thickness of 20 nmfor the barrier 425. Note that these values are highly variable invarious implementations. More generally, it is contemplated that thethickness values can range between approximately 10 and 400 nm for theenergy gap depletion region layer 330/430 thickness and betweenapproximately 5 and 200 nm for the barrier 425.

The illustrative depletion region 430 depicted in FIG. 4 is composed oftwo discrete layers. However, those skilled in the art will comprehendthat the number of layers can be varied in alternate embodiments withinthe general parameters in which at least two different energy gapmaterials are employed in the construction of the device. Any number oflayers can be used in the depletion region, of any absolute energy gapvalue, with the relative energy gaps typically conforming to thespecifications given above (i.e. consists of a material with a higherenergy gap than an energy gap of a material that defines the base) andmore generally contemplated herein. It is also expressly contemplatedthat compositional grading between layers can provide benefits for bothenhanced photo-generated carrier escape and enhancements in the built-inelectric field. The use of materials forming type II heterointerfacescan also provide additional benefits to the material structure.

The illustrative material structure depicted in FIG. 4 shows an n-typeemitter over a p-type base, with wide band gap material in the depletionregion adjacent to the n-side suppressing electron injection. Thoseskilled in the art will readily recognize that an essentially equivalentdesign consists of a p-type emitter over a n-type base, with wide bandgap material in the depletion region adjacent to the p-side suppressinghole injection. An alternate implementation, based upon the structuresdepicted in FIGS. 3 and 4 also includes a material structure with wideband gap barrier layers adjacent to either the p-side or the n-side orboth.

The illustrative embodiments in FIG. 5 depict specific materialstructures for an extended heterojunction with barrier solar cell orsubcell as applied to InGaP-based materials, which are often used forthe top subcell in III-V multijunction solar cells. Three embodimentscovered by respective top windows 500, 500 a and 500 b are depicted withrespect to the associated graph of energy versus position (depth). Byway of example, the following parameters can be employed in the firstembodiment: an InGaP base layer 540 approximately 1000 nm in thicknesspositioned over a back surface 560. The base layer 540 contains anapproximately 100 nm emitter 520 and a 200 nm energy gap depletionregion layer with barrier layer 525/530. In this implementation ofmaterials, the energy gap of the emitter 520 and within the depletionregion 530 can be increased by adding Al to form AlInGaP alloys,increasing the Ga content in the InGaP, or forming a disordered InGaPalloy. Another illustrative embodiment, having window 500 a, can be madestrain-free relative to an underlying GaAs or Ge substrate, AlInGaPalloys are employed for both the emitter 520 a and the higher energy gapdepletion layer 530 a, such that the barrier layer 525 a containsmaterial with higher Al compositions. In this embodiment, disorderedInGaP is employed in the higher energy gap depletion region 530 aadjoined to the base 540 a, while the base 540 a itself is composed ofordered InGaP, which has a lower energy gap. The base 540 a ispositioned over a back surface 560 a. In another embodiment havingwindow 500 b, which can be made free of Al, Ga-rich InGaP is employed inthe emitter 520 b and barrier layer 525 b. While this approachintroduces lattice strain, the strain can be compensated by grading to alower Ga-composition InGaP alloy through the high E_(g) depletion regionlayer 530 b and into the base 540 b. The illustrative base 540 b ispositioned over a back surface 560 b.

The illustrative embodiments in FIG. 6, each aligned with a common graphof energy versus position, depict specific material structures for anextended heterojunction with barrier solar cell or subcell as applied toGaAs materials, which are often used for the middle subcell in III-Vmultijunction solar cells. By way of example, the following parameterscan be employed in the embodiment covered by window 600: a GaAs baselayer 640 approximately 3000 nm in thickness, with an approximately 100nm emitter 620 and a 200 nm energy gap depletion region layer withbarrier layer 625/630. In this embodiment, the energy gap of the emitter620 and within the depletion region 630 can be increased relative to theGaAs base 640 by employing InGaP or AlGaAs alloys. The back surfacefield layer 660 is shown, positioned opposite to the window 600. In adesirable configuration covered by window 600 a, an InGaP latticematched to GaAs is employed for the emitter 620 a, while AlGaAs alloysare used for the higher energy gap depletion layer 630 a, such that thebarrier layer 625 a contains material with higher Al compositions. AGaAs base 640 a is provided in this embodiment. Compositionally gradedAlGaAs alloys can be used to smooth the transition to GaAs within thedepletion region. In an alternate embodiment covered by window 600 b,AlGaAs alloys can also be employed in the emitter 620 a. Layers 630 b,640 b and 660 b are similar to their respective counterparts 630 a, 640a and 660 a, described above.

The illustrative embodiments in FIG. 7 depict specific materialstructures for an extended heterojunction with barrier solar cell orsubcell as applied to InGaAs materials, which are often used for thelower subcell in III-V multijunction solar cells. These layers aredescribed in conjunction with a common graph of energy versus position.Embodiments covered by respective windows 700, 700 a and 700 b aredepicted in alignment with the energy band graph. In one embodiment, thefollowing parameters can be employed: an InGaAs base layer 740approximately 3000 nm in thickness, with an approximately 100 nm emitter720 and a 200 nm energy gap depletion region layer with barrier layer725/730. A back surface field 760 is provided opposite the window 700.When InGaAs alloys are used, the energy gap of the emitter 720 andwithin the depletion region 730 can be increased by employing InGaPalloys. In a further desirable configuration (covered by window 700 aand including opposing layer 760 a), InGaP lattice matched to the InGaAsbase 740 a is employed for the emitter 720 a and higher energy gapdepletion layer 730 a, while lattice matched AlInGaP alloys are used thebarrier layer 725 a. In another alternate embodiment (having window 700b and opposing layer 760 b), the barrier layer 725 b contains materialwith higher Ga compositions, and is strained relative to the InGaAs base740 b. Other layers 720 b, 730 b are generally similar to thosecounterparts described above.

Undesired reflection of incident photons from the top surface of asingle junction or multijunction solar cell can be minimized by theincorporation of transparent antireflection coating structures. Thesecoating can be implemented in accordance with industry standardprocesses and materials in various embodiments. Antireflection coatingstypically employ single or multiple layers of materials with index ofrefraction intermediate between the semiconductor and the media in whichthe incident photons are delivered (often air). Conventionalsingle-layer antireflection coatings, although widely used, typicallyoperate only at a single wavelength and at normal incidence.Graded-index coatings with variable-index profiles have beeninvestigated for broadband antireflection properties, particularly withair as the ambient medium. For example, previous modeling work hassuggested that a quintic-index profile is a near optimum profile for agraded-index antireflection coating (see, for example, U.S. Pat. No.4,583,822, entitled QUINTIC REFRACTIVE INDEX PROFILE ANTIREFLECTIONCOATINGS, by W. H. Southwell, the teachings of which are expresslyincorporated herein by reference as useful background information).

Oblique-angle deposition has recently been demonstrated as an effectivetechnique for tailoring the refractive index of a variety of thin filmmaterials (see for example, by way of useful background, J.-Q. Xi, M. F.Schubert, J. K. Kim, E. F. Schubert, M. Chen, S.-Y. Lin, W. Liu, and J.A. Smart, Optical Thin-Film Materials with Low Refractive Index forBroad-Band Elimination of Fresnel Reflection, Nat. Photon., vol. 1, pp.176-179, 2007). Oblique-angle deposition is a method of growingnanostructured, porous thin films, and hence thin films withlow-refractive index (low-n), enabled by surface diffusion andself-shadowing effects during the deposition process. Both conductingand non-conducting graded-index antireflection coatings that arebroadband and Omni-directional have been demonstrated using thisdeposition technique. As taught by Cho et al. in U.S. Pat. No.7,483,212, both oblique angle deposition and co-sputtering are materialsynthesis techniques that can be used to construct multiple layer,graded refractive index coatings to minimize reflection losses. Theteachings of this patent are expressly incorporated herein by referenceas useful background information. It is contemplated in illustrativeembodiments that these processes can be adapted to minimize reflectionlosses in epitaxial III-V single- and multi-junction solar cells.

FIG. 8 depicts a desirable antireflection coating for a III-Vmultijunction solar cell. In particular, the index of refraction (n)should be varied from that of the InGaP 840 used in the base layer ofthe top subcell (approximately 3.6) to that of air (e.g. near 1). Acontinuously varying quintic profile 800 of the index of refraction,such as that taught in U.S. Pat. No. 4,583,822, can be approximated by astep graded profile 820. In the design shown in FIG. 8, the index ofrefraction is varied from 3.6 to 1.1 over four discrete steps, which canconsist of 100 nm of TiO₂ 860 (n˜2.65), 120 nm of TiO₂/SiO₂ 870(n˜2.05), 125 nm of TiO₂/SiO₂ 880 (n˜1.55), and 365 nm of porous SiO₂890 (n˜1.1).

One deficiency of the graded refractive index (GRIN) antireflectionstructure shown in FIG. 8 is the large index of refraction step betweenTiO₂ 860 (n˜2.65) and InGaP 840 (n˜3.6). The multilayered structure inFIG. 9 addresses this deficiency by employing epitaxial semiconductorlayers between the InGaP and the TiO₂. In particular, the index ofrefraction in varied from 3.6 to 1.1 over six steps in a graded profile920 to approximate a quintic profile 900, with approximately 70 nm ofAlInGaP 945 (n˜3.3), 90 nm of AlInP 955 (n˜3.0), 100 nm of TiO₂ 960(n˜2.65), 120 nm of TiO₂/SiO₂ 970 (n˜2.05), 125 nm of TiO₂/SiO₂ 980(n˜1.55), and 365 nm of porous SiO₂ 990 (n˜1.1).

The illustrative antireflection structures depicted in FIGS. 8 and 9show particular combinations of layer thicknesses and index ofrefractions. However, those skilled in the art will comprehend thatessentially equivalent designs consist of structures with any number oflayers, and with a range of individual layer thicknesses and index ofrefractions. In particular, the novel use of epitaxial semiconductorlayers to grade the index of refraction from the top subcell to thenon-epitaxial optical material layers (e.g. TiO₂ or SiO₂), inillustrative embodiments, provides effective performance. It should befurther noted that epitaxial semiconductor layers such as AlInGaP andAlInP can be designed to simultaneously function as window and contactlayers in III-V single- or multi-junction photovoltaic devices.

FIG. 10 is a cross sectional view illustrating a multijunction solarcell with two epitaxial subcells incorporating the previously describeddesign elements for enhancing both the voltage and current output of thedevice. Fabrication of this device begins by placing a commerciallyavailable Ge or GaAs substrate 1000 into a standard III-V semiconductordeposition tool, such as a metal organic chemical vapor deposition(MOCVD) system available (for example) from Aixtron AG of Germany orVeeco Instruments of Plainview, N.Y. After then depositing a suitablebuffer and/or transition layers 1005 on the substrate, a lower subcell1010 is formed, followed by tunnel junction 1045 and, as required,grading layers, followed by a top subcell 1050, and finally suitablecontact layers 1092. The top subcell 1050 consists of an AlInP window1090 and an AlInGaP emitter 1080. In a desirable embodiment, thethickness and index of refraction of these top AlInP and AlInGaP layersare designed to also function as part of the antireflection structure asillustratively depicted in FIG. 9. Higher energy gap material may beincorporated into the depletion region of the top subcell 1050, and inparticular, AlInGaP in the barrier layer 1080 and disordered InGaP asthe remaining extended heterojunction layer 1075. An ordered InGaP baselayer 1060 and AlInGaP back surface field (BSF) 1055 are employed in thetop subcell 1050 of the example shown in FIG. 10. The lower subcell 1010consists of an AlInP window 1040 and an InGaP emitter 1035. AlGaAsalloys are then used for the barrier layer 1030 and extendedheterojunction 1025 as in FIG. 6. Either GaAs or InGaAs can be employedin the base layer, depending on the substrate and nature of the overallstructure (e.g. lattice matched to the substrate or metamorphic). Afterthe epitaxial deposition step, a photovoltaic device is formed viastandard semiconductor processes to create top and bottom metalcontacts, and unwanted epitaxial top contact material 1092 isselectively removed. As a final manufacturing step, optical thin filmcoatings are deposited via physical deposition methods such asco-sputtering and oblique angle deposition. The thickness and indexproperties of these coatings in a desirable embodiment follow the designdepicted in FIGS. 8 and 9.

FIG. 11 is a cross sectional view illustrating a multijunction solarcell with three epitaxial subcells incorporating the previouslydescribed design elements for enhancing both the voltage and currentoutput of the device. Fabrication of this device begins by placing acommercially available GaAs substrate 1100 into a standard III-Vsemiconductor deposition tool (MOCVD or MBE). After then depositingsuitable buffer and release layers 1105 on the substrate, the cell isgrown in an inverted fashion by first forming a top subcell 1112,followed by tunnel junction 1140 and, as required, grading layers,followed by a middle subcell 1144, another tunnel junction and gradinglayers 1172, a lower subcell 1176, and finally suitable contact layers1194. The top subcell 1112 can consist of an AlInP window 1116 and anAlInGaP emitter 1120. In an exemplary implementation, the thickness andindex of refraction of these top AlInP and AlInGaP layers are designedto also function as part of the antireflection structure asillustratively depicted in FIG. 9. Higher energy gap material may beincorporated into the depletion region of the top subcell 1112, and inparticular, AlInGaP alloys in the barrier layer 1124 and extendedheterojunction layer 1128. An InGaP base layer 1132 and AlInGaP backsurface field 1136 are employed in the top subcell 1112 of the exampleshown in FIG. 11. The middle subcell 1144 consists of an AlInP window1148 and an InGaP emitter 1152. AlGaAs alloys are then used for thebarrier layer 1156 and extended heterojunction 1160. GaAs is employed inthe base layer 1164, and AlGaAs as the back surface field 1168. Thelower subcell 1176 consists of an InGaP window 1180, emitter 1184,barrier layer 1186, extended heterojunction layer 1188, and back surfacefield 1192. The energy gap can be varied by changing the Ga compositionof the InGaP alloy. In a typical inverted metamorphic structure (see forexample, Wanlass, US Published Application No. 2006/014435), the InGaPemitter composition is In-rich to match the lattice constant of theInGaAs base 1190. In the improved inverted metamorphic structure shownin FIG. 11, higher energy gap InGaP is formed in the depletion region byincreasing the Ga content. After the epitaxial deposition step, aconductive carrier 1196 is attached to the epitaxial layer stack and thesubstrate with release layers 1100 removed. A photovoltaic device isthen formed via standard semiconductor processes to create top andbottom metal contacts and selectively remove unwanted epitaxial topcontact material 1104. As a potential completion step in the solar cellmanufacturing process, optical thin film coatings can be deposited viaphysical deposition such as co-sputtering and oblique angle deposition(and/or other conventional techniques). The thickness and indexproperties of these coatings in an exemplary embodiment follow thedesign depicted in FIGS. 8 and 9.

The many features and advantages of the illustrative embodimentsdescribed herein are apparent from the above written description andthus it is intended by the appended claims to cover all such featuresand advantages of the invention. Further, because numerous modificationsand changes will readily occur to those skilled in the art, it is notdesired to limit the invention to the exact construction and operationas illustrated and described. For example, the illustrative embodimentscan include additional layers to perform further functions or enhanceexisting, described functions. Likewise, while not shown, the electricalconnectivity of the cell structure with other cells in an array and/oran external conduit is expressly contemplated and highly variable withinordinary skill. More generally, while some ranges of layer thickness andillustrative materials are described herein. It is expresslycontemplated that additional layers, layers having differing thicknessesand/or material choices can be provided to achieve the functionaladvantages described herein. In addition, directional and locationalterms such as “top”, “bottom”, “center”, “above” and “below” should betaken as relative conventions only, and not as absolute. Accordingly,this description is to be taken only by way of example and not tootherwise limit the scope of the invention.

What is claimed is:
 1. A solar cell device comprising: a subcell-basedstructure that includes a topmost subcell; and a broadband, step-gradedantireflection structure located above the topmost subcell, theantireflection structure comprising a plurality of deposited layers ofdiscrete materials comprising: a layer of approximately 70 nm of AlInGaPhaving an index of refraction of approximately 3.3, a layer ofapproximately 90 nm of AlInP having an index of refraction ofapproximately 3.0, a layer of approximately 100 nm of TiO₂ having anindex of refraction of approximately 2.65, a layer of approximately 120nm of TiO₂ and SiO₂ having an index of refraction of approximately 2.05,a layer of approximately 125 nm of TiO₂ and SiO₂ having an index ofrefraction of approximately 1.55, and a layer of approximately 365 nm ofporous SiO₂ having an index of refraction of approximately 1.1.